I am not a dev, just a user, so I have no say in what does or does not get implemented. However features such as PINs, passwords and local encryption have been raised many times both here and on GitHub. The below are just two examples:
In summary, at this time Joplin does not offer any local encryption features as it is far more effective to use encryption at the OS level to secure all data on the device.
Even so @laurent, the project maintainer, has previously stated that he would still consider a pull-request should someone come up with a suitable implementation. It's just not on Joplin's "roadmap".